1. Field of the Invention
The present invention relates to a path routing computation method and an optical communication network applying the path routing computation method. Particularly, the present invention relates to a path routing computation method for setting a path in a network connected to wavelength convertible and inconvertible subnetworks constituted by Reconfigurable Optical Add/Drop Multiplexing (ROADM) devices.
2. Description of the Related Art
Regarding the forms of optical communication networks, there is a ROADM ring network in which ROADM devices present in nodes are connected to form a ring. In the example shown in FIG. 1 a ROADM ring network is constituted such that three ROADM rings 1, 2, and 3 are connected with each other as subnetworks. The inside of each of the ROADM rings is fixed to a transmission wavelength for fixation. Specifically, wavelengths are fixed so that wavelength conversion cannot be performed.
FIG. 2 is a configuration example of a ROADM device having, for example, a ring 1. The inside of the ring 1 is constituted such that a wavelength signal with a wavelength of λ1 through λ10 is transmitted, and that an optical signal having a specific wavelength is added and dropped from or to a tributary portion 13 through a switch 12a, 12b. 
Therefore, in FIG. 1, when an optical signal with a wavelength 1 is transmitted from a ROADM device of a node A as a starting point to a ROADM device of a node Z as an end point, the inside the ring 1 is transmitted at the fixed wavelength 1 as it is, and then the wavelength 1 is converted at a node in the ring 3 and transmitted at a wavelength 5 fixed inside the ring 3.
In this manner, the ROADM ring network has a restriction in the transmission wavelengths, and further has a restriction in transmittable wavelengths based on deterioration and the like in transmission of signals in the nodes through which the signals pass.
On the other hand, as other form of an optical communication network, there is a WDM network in which paths are connected in the form of a mesh. This is a wavelength convertible network which does not have a restriction that the transmission wavelengths are fixed as in the ROADM ring network shown in FIG. 1 above (non-blocking).
FIG. 3 shows a network in which wavelength inconvertible ROADM ring networks 101 and 102 as subnetworks are connected to a wavelength convertible WDM network 100 described above.
In the networks shown in FIG. 1 and FIG. 3, a routing path needs to be set up in order to transmit an optical signal from a starting point node to a destination (end point) node.
In order to set up a routing path, a routing protocol is used beforehand so that network information such as limiting conditions is notified between the nodes, and the network information such as the notified limiting conditions and costs (a random criterion which is set by a user in advance, a numeric value based on, for example, the path selection priority, and the like) between the nodes is stored in a routing table provided in a ROADM device of each node.
In a distributed processing system, in a ROADM device of a node as the starting point which receives a path set-up request, path routing computation for obtaining a path MSL where a routable cost is the minimum is carried out on the basis of the information stored in the routing table.
Alternatively, in a centralized data processing system, network information is stored in a network management system (NMS) for centrally controlling the states of the nodes on the network. On the basis of such network information, the NMS, which receives a request via the ROADM device of the starting point node, performs path routing computation.
As a conventional technology related to path routing computation, there is the invention described in Japanese Patent Application Laid-Open No. 2003-198609. An object of the invention described in Japanese Patent Application Laid-Open No. 2003-198609 is to perform routing computation which does not fail by depending on the restriction on the connectivity between cross-connect devices in an optical communication network.
Further, a virtual neighbor link is defined in backbone, and an optical path routing between a starting point and an end point is computed using the information on the virtual neighbor link.
Moreover, the information described in Japanese Patent Application Laid-Open No. 2002-291372 describes a path routing computation system in which is used a boundary node for performing path setting.
Now, suppose that there is a network configuration as shown in FIG. 4. In this network, ROADM rings R1 through R6 are connected to a mesh network 100 in a multi-stage manner. In such a configuration, the mesh network 100 does not have restrictions in wavelengths. On the other hand, the ROADM rings R1 through R3 and R4 through R6 connected to the mesh network 100 are, as described in FIG. 1, have restrictions in wavelengths and the like.
Regarding path routing computation for path setting, for the ring network in which are connected the ROADM rings R1 through R3 on the starting point node A side, which are connected to the mesh network 100, it is only necessary to consider only a starting point wavelength of λ1 from the starting point node A, as in the explanation of FIG. 1.
However, in the plurality of ROADM rings R4 through R6 on the output side of the mesh network 100, a plurality of wavelengths need to be considered. In this case, the output side of the mesh network 100 has paths in usable wavelengths between λ5 through 15 and in usable wavelengths between λ12 through 20, thus a candidate wavelength cannot be specified.
It is necessary to determine which one of the transponders of the ROADM devices is selected and used to obtain a path of the shortest distance or the lowest cost.
In a method for such a purpose, it is necessary to generate a wavelength graph for each wavelength and search for the shortest distance on the basis of the wavelength graph. The wavelength graph is information obtained by using a wavelength fixed for each link, to define a connection between the links. Therefore, a wavelength graph corresponding to each wavelength needs to be created. Furthermore, generally a scale required for searching for a path is considered to be proportional to the square of the number of nodes. Therefore, there is a problem that the memory capacity required for routing computation becomes extremely large if the number of multiplexed wavelengths increases.